Apparatuses are known in which food products are disposed in closed or open containers which are permeable to high-frequency radiation. The containers are located on an endless conveyor belt which conveys them through an elongate treatment chamber. High-frequency radiation emitters are introduced into the treatment chamber from above and below, that is to say, perpendicularly to the longitudinal axis of the chamber. Temperature sensors are disposed both upstream and downstream of the emitters. These sensors are connected to a computer, which latter is also connected to the conveyor belt arrangement so that, effectively, the amount of irradiation received by the food products is controlled.
German Patent Specification No. DE 3432341 discloses pasteurising apparatus, which is generally of the above-described type and in which microwaves are used as the high-frequency radiation. The irradiation treatment is carried out in three sequential treatment chambers and the microwave energy is reduced, in a stepwise manner, as the three chambers are successively traversed by the food. If the product to be treated is, for example, a pre-cooked meal and is to be subjected to a pasteurising process, it is very important to achieve a uniform temperature for all of the constituents of the meal. It is very difficult to meet this requirement, because each individual constituent requires a different amount of heat to achieve a uniform temperature.
In the prior art methods, the standard treatment temperature of the product has been taken as that of the constituent which first reaches its pasteurising temperature. The upper treatment temperature limit is the temperature which cannot be exceeded if harm to one or more constituents by excessive heating is to be avoided. In such circumstances, it often occurs that some components of the meal, such as meat in gravy, do not, in fact, reach their pasteurising temperature. This is because any further increase in the overall temperature, which would be necessary to pasteurise the meat and gravy components would necessarily have caused the other constituents of the meal to be harmed.
In German Patent Application No. P38 34 574.9-45 another apparatus of the above-described general type is disclosed. In such apparatus temperature sensors, which measure the temperature of the individual components in the containers are disposed immediately upstream of each supply conduit of a microwave emitter. The supply conduit projects vertically into the treatment chamber either from below or from above the conveyor belt. A computer compares the measured temperatures with stored characteristic values specific to the products and the output of each microwave emitter is controlled in dependence upon the comparative figures.
German Patent Specification No. DE 3 806 816 also discloses an apparatus for pasteurising or sterilising food utilising microwaves. The microwave emitters used in such apparatus are microwave generators which communicate with supply conduits, which latter define the particular microwave beam. In such a case, the geometry of the configuration of the cross-sectional area of the discharge aperture of the microwave supply conduits corresponds substantially to the projection geometry of the products to be treated when viewed in the direction of the conveyor belt.
Finally, European Patent Specification No. 0 128 397 discloses a capacitive high-frequency continuous heating oven, wherein products which are relatively long but not very thick, such as textile webs may be heated in a particularly advantageous manner. The continuous heating oven disclosed in such specification comprises a high-frequency generator, which induces vertically adjustable plate or extrusion electrodes to produce electromagnetic waves having a frequency in the range of, for example, 30 MHz. In addition, an earthed counterelectrode is provided, which comprises a grid of earthed stray field electrodes, which are disposed vertically relative to the direction of conveyance of the web.
The prior art apparatuses which utilise ultrahigh-frequency technology, that is to say, microwave technology are disadvantageous on the ground of costs, because one microwave emitter has to be provided at each irradiation position. In addition, the microwave radiation lobe can generally only be adjusted to the projection geometry of the product by screening-off areas which are not to be irradiated.
A known apparatus for heating products by means of a source of high-frequency radiation is in fact produced more simply and more economically by utilising emitter technology. However, it is not suitable for the differentiated irradiation adapted to the geometry and the constants of the constituents of a product because a radiation field having a substantially rectangular, flat cross-section is produced.